(1) Field of the Invention
The invention relates to the general field of etching by non chemical means, particularly RF sputter-etching.
(2) Description of the Prior Art
RF (radio frequency) sputter-etching is a universal etching technique, although the actual rate at which material gets removed does vary from one material to another. DC (direct current) sputter-etching is similar, but is limited to electrically conductive material, which RF sputter-etching is not. The basic setup is to provide two electrodes—a substrate holder and a sputtering shield, immerse them in gas at low pressure and apply an RF voltage between them.
FIG. 1 shows an exploded isometric view of part of a standard sputter-etcher of this type, in this case Electrotech's model MS6210 which was used in the development and reduction to practice of the present invention. During use, silicone oil cooled substrate holder 1 (shown in FIG. 2) is positioned just inside the open lower portion of sputtering shield 2. Air cooled lid 4 is attached to 2 along with vacuum tight O-ring seal 3. The various attachments labelled as 5 include means for admitting the sputtering gas through lid 4 and controlling its pressure, as well as means for applying the RF power. Gas baffle 11 is also shown.
Not shown in FIG. 1 is a vacuum chamber within which the sputter etcher resides during use. An appropriate sputtering gas (usually argon) is admitted into said vacuum chamber during use, its admission rate being adjusted so that some particular desired pressure level can be maintained. Said pressure is commonly about 6×10−3 torr. RF voltage is applied between the substrate holder and the sputtering shield, initiating an RF glow discharge, and material is removed for both electrodes at a rate that is inversely proportional to some power (between 1 and 2, depending on the exact geometry) of the ratio of their two areas. See, for example, Koenig in U.S. Pat. No. 3,661,761 May 9, 1972. Since the area of the substrate holder is, by design, significantly less than that of the sputtering shield, the rate at which material will be removed from the substrate (assumed to be covering almost the entire substrate holder surface) will be substantially greater than the rate of material removal from the sputtering shield.
Sputter-etchers of the type illustrated in FIG. 1 are intended for the sputter-etching of a single semiconductor wafer at a time. This allows for better control of the amount of material removed from different wafers than is possible in a batch system which processes many wafers at a time. However, in order to make the throughput of such a system economically attractive in a manufacturing environment, it is necessary that the time taken to etch a given wafer be kept as low as possible. This implies that significantly higher etch rates, and therefore significantly higher levels of RF power density, must be used relative to the batch methods. To avoid the need for very high RF voltages to achieve these high power densities (typically about 300 watts per wafer) these single wafer etchers operate at higher gas pressures (where plasma resistance is lower) than do the batch units.
This use of high pressure, and particularly high power, is known to have certain undesirable side effects, notably the generation of fine dust particles, some of which find their way onto the surface that was sputter-etched. Any particulate matter of this type, if allowed to settle on the surface of an integrated circuit during the course of its manufacture, has a high probabilty of destroying said circuit, thus reducing the overall product yield.
The solution to this problem, which forms the subject matter of the present invention, involves the coating of the inside wall of the sputtering shield with a layer of material that has a rough surface, such as that obtained when said layer has been deposited through the method of arc-spraying. The use of arc-sprayed material in connection with integrated circuits has been described by Narasimhan et al. in U.S. Pat. No. 4,320,251 Mar. 16, 1982. Said invention relates to the formation of ohmic contacts in solar cells (using arc-sparaying) and does not relate in any way to sputter-etching.
The exact reasons for the efficacy of the present invention are not fully understood and it is likely that more than one physical mechanism is involved. One of these possible mechanisms is that the rough surface associated with layers deposited through arc-spraying is better able to trap material originating at the substrate and prevent its re-emission and eventual return to the substrate. The ability of small receivers of a particular geometrical shape to suppress re-emission during sputter deposition was described by Maissel et al. in the IBM Jour. of R&D vol. 14 p. 176 in March 1970. However, this is only a very general teaching and does not suggest the detailed present invention, even assuming that the suppression of re-emission is the only physical mechanism involved.